The present invention relates generally to metalworking fluids and, more particularly, to methods of using additives to regenerate and stabilize metalworking fluids and to diminish dermatitis and other health problems associated with the use of metalworking fluids. This invention also relates to a wholly or partially automated system for implementing these methods.
In the fabrication of metal parts by machining and by various cold forming processes, a lubricant is often used. In a machining operation that involves cutting or other removal of material from the workpiece, the lubricant aids in removing heat generated by engagement of the cutting tool, in removing chips, turnings, and fines generated during such machining, and in lubricating or protecting newly exposed part surfaces. In cold forming operations, a lubricant may be applied to the slug surface to decrease friction during deformation and to lubricate newly exposed surfaces. For cutting operations, the fluid may be circulated continuously over the workpiece.
Especially for turning and similar lathe operations, the lubricant is typically called a "metalworking fluid." Metalworking fluids (MWFs) are used throughout the manufacturing industry to provide a more efficient material removal or forming operation. Such fluids are selected generally for the purpose of cooling the workpiece and the tool during cutting operations, and to facilitate removal of chips during turning, grinding, and similar operations. MWFs are an important facet of many manufacturing operations in that they provide the required chip and heat removal properties necessary to achieve higher production outputs, increased tool life, and enhanced machined-surface finish and part quality.
MWFs are generally categorized as oil-based, synthetic, or semisynthetic. Oil-based MWFs may be comprised of straight (neat) oils derived from petroleum and/or paraffinic hydrocarbons, natural oils such as vegetable oils, or may be comprised of a synthetic oil such as a siloxane or dibasic ester. Water-based MWFs may comprise a water-soluble oil, a synthetic composition, or a semisynthetic composition. Water-based synthetic fluids are free of mineral oils, and their aqueous environment can spawn bacterial and/or fungal growth. Semisynthetic fluids and water soluble oils generally combine the advantages of emulsifiable oils and synthetic fluids, and they are typically provided as an oil-in-water emulsion. See generally, T. Cole, "Know Your Coolants," Cutting Tool Eng., October 1990, p. 59f.
Conventional additives for MWFs include corrosion inhibitors, such as alkaline and alkanolamine salts of organic acids, sulfonates, amines, amides, organic borate compounds, and others. Corrosion inhibitors are generally added to the hydrophobic portion of the MWF. Polar compounds, such as certain animal or vegetable oils (e.g., castor oil, olive oil, peanut oil), esters of organic acids, and the like are considered to be chemically inactive MWF ingredients. Polar additives function through electrostatic attraction in which the small polar group adsorbs onto the metal surface, and the larger hydrophobic group is solubilized by the oil phase. These characteristics produce a securely anchored mono-molecular film (a metallic soap) on the exposed metal surface that functions as a protective barrier.
"Extreme pressure" additives are another type of conventional additive and typically consist of chlorine, phosphorous, or sulfur compounds. These additives may be used in heavy-duty machining operations where the temperature is greater than that tolerated by polar additives, that is greater than about 390.degree. F. (about 200.degree. C.). Other conventional additives include solubilizers (e.g., long-chain alcohols), colorants, fragrances, anti-oxidants, anti-foaming agents, and the like. See, e.g., Keith Bienkowski, "Waste Minimization Through Improved Coolant Management," Technical Paper No. MRR92-12, Soc. of Mfg. Eng., Dearborn, Mich. (1992).
MWFs are susceptible to microbial attack by bacteria, fungi, and/or yeasts (collectively termed herein "microbes"), causing one or more symptoms such as odor development, a decrease in pH, a decrease in dissolved oxygen concentration, changes in emulsion stability (for water soluble oils and semisynthetic fluids), increased incidence of dermatitis, workpiece surface-finish blemishes, clogged filters and lines, increased workpiece rejection rates, decreased tool life, and generally unpredictable changes in coolant chemistry. See, e.g., Frederick J. Passman, "Microbial Problems in Metalworking Fluids," Lubrication Engineering, pp. 431-3, May 1988. See also I. Mattsby-Baltzer et al., "Microbial Growth and Accumulation in Industrial Metal-Working Fluids," Applied and Environmental Microbiology, October 1989, pp. 2681-2689.
Biocides are typically added to the MWF to combat microbial contamination. One conventional biocide is o-phenylphenol (available as DOWICIDE 1 antimicrobial fluid from Dow Chemical USA, Midland, Mich.). Hernandez (Canadian Pat. No. 1,161,026) describes inherently bactericidal MWFs that include a mixture of boric acid, an alkali tetraborate, pelargonic acid (also known as normal ennoic acid or nonanoic acid, CH.sub.3 (CH).sub.7 COOH), a nonionic surfactant, and water. Biocides typically have a limited lifetime of only a couple or a few days and, accordingly, are replenished weekly if not daily according to conventional manufacturing protocol.
While reported opinions differ as to whether MWFs can transmit communicable disease, and regarding the general viability of microbes in MWFs, it is fairly well-established that used MWFs are associated with outbreaks of dermatitis. Two potential sources of MWF-induced dermatitis are compounds intentionally added to the MWF (such as biocides) and solubilized metal ions from metal chips, dust, and fines, typically collectively referred to as "swarf", deposited in the MWF by the machining or forming operation.
Metal ions can become solubilized in the MWF due to the generation of heat, friction, oxidation, and other chemical and physical processes which occur in the MWF environment. Virtually any metal ion, including iron, nickel, chromium, cobalt, cadmium, copper, manganese, or zinc may cause contact dermatitis in a particular machine operator. Iron, nickel, and chromium are typically found in swarf generated from machining various types of stainless steel. Cobalt is typically encountered when machining with cutting tool steel and tungsten carbide cutting tools.
Bacteria are known to use metal ions in their metabolism. Bacterial metabolism is the typical cause of underarm odor, and so deodorants typically include an antibacterial agent (e.g., benzalkonium chloride, triclosan, etc.). See also Warren P. Iverson, "Mechanism of Anaerobic Corrosion of Steel by Sulfate Reducing Bacteria," Nat'l Assoc. of Corrosion Eng., Paper No. 83-243 (1984). Anaerobic sulfate-reducing bacteria metabolize sulfur-containing compounds present in the MWF introduced from the water source, cutting oil additives, and other contaminants which enter the cooling fluid system. The metabolic product of these sulfur-reducing bacteria is typically hydrogen sulfide, an odious compound.
In spite of the existence of a wide variety of antimicrobial compounds useful for treating MWFs, there are still significant problems associated with microbial growth in MWFs. These problems range from those mentioned above, such as dermatitis and poor workpiece surface finish, to noisome conditions and corrosion of the metalworking machinery. The standard industry practice of recycling MWFs leads to an accumulation of solubilized metals at ever increasing concentrations. See "Dermatitis Is a Key Consideration When Selecting a Coolant," Lubricants World, July 1992, p. 8. The alternative of frequent replacement of MWFs is costly due to environmental problems associated with disposal.
The foregoing illustrates limitations known to exist in present MWFs and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.